Add defaults for expr and description to prevent spurious diffs

[modular-magician]

Fixes: #9084

Probably.

Problem sort of explained here (#9084 (comment)) where the state value seems to be changing between null & "" which causes the diff on the rule. Setting default values for both description & versioned_expr seems to fix this as far as I can tell

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Release Note Template for Downstream PRs (will be copied)

compute: fixed extra diffs generated on `google_security_policy` `rules` when modifying a rule

Derived from GoogleCloudPlatform/magic-modules#5997

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